In the double seat valve known from EP 1 730 430 A, the two valve disks have ring grooves having different cross-sections and different ring seals in the ring grooves. In the one-piece valve disk providing a seat valve function, an integrally formed sealing ring of a trapezoidal shape in its cross-section is incorporated such that its external sealing zone is oriented approximately axially and towards the other valve disk. In the ring groove of the valve disk providing a sliding valve function, a composite ring seal is arranged which consists of an elastic sealing ring having an essentially C-shaped cross-section and a metallic backup ring having an approximately T-shaped cross-section and being connected with the sealing ring by vulcanization in a material connection. The two sealing rings at least have different shapes and dimensions. Parts of the valve disk which are screwed together axially support themselves at the vertical web of the T-cross-section of the backup ring. The sealing ring for the sliding valve function has an essentially radially oriented external sealing zone and another central sealing zone adjacent to it which is essentially oriented axially. Due to the two sealing zones, the preload frictional connection of the sealing ring in the ring groove is defined such that in the dynamic sealing function, one cannot reliably exclude that anything will get behind the sealing ring in the ring groove. Moreover, the composite ring seal is complicated and expensive to manufacture, and when the sealing ring is replaced, the backup ring must be replaced, too. The provision of two different ring seals for the double seat valve involves additional logistic efforts. The assembly of the sealing ring for the seat valve function is difficult, involves extreme deformation and the risk of product getting behind the sealing ring and the seal being torn out. As in the closed position of the valve disk responsible for the seat valve function there is no clear metallic stop of the valve disk under extreme process conditions, extreme and varying loads on the sealing ring result. The sealing rings are expendable parts that have to be replaced when the sealing effect is reducing.
In a double seat valve known from DE 101 47 455 C of another type, that is with two valve disks having the same diameters but only providing a sliding valve function each, the same ring seals are mounted in the two valve disks each for an exclusively radial sealing effect. The two valve disks consist of at least two parts. The screwed parts together limit the ring groove for the ring seal. Each ring seal consists of three components, that is a dimensionally stable backup ring having a C-shaped cross-section, a restoring ring of an elastic material positioned in the backup ring, for example an O-ring, and the sealing ring consisting of elastic material, which has a C-shaped cross-section and is placed onto the backup ring such that it can be exchanged. As the backup ring is only radially supported at at least one part of the valve disk, but the parts of the valve disk are mutually clamped so as to contact each other, the clamping width of the ring groove responsible of the preload form-fit of the sealing ring in the ring groove is inappropriately influenced by inevitable manufacturing tolerances of the parts of the valve disk, so that within one series of valve disks, different preload frictional connections of the sealing rings cannot be excluded. For an easier assembly, the backup ring can be assembled from segments. Between the sealing ring and the restoring ring as well as the backup ring, in a dynamic sealing function of the sealing ring in the seat, inevitable relative motions with locally concentrated friction and stress occur which endanger the service life of the sealing ring and the sealing function.